Wave–particle duality

Wave–particle duality is the principle that quantum objects display both wave-like and particle-like behavior. Which behavior appears depends on the experimental arrangement and measurement.

Double-slit experiment: An experiment in which particles or waves pass through two slits, producing interference under suitable conditions. It demonstrates interference from quantum particles and the loss of that pattern when path information is measured.
Quantum mechanics: The physical theory describing matter and radiation at atomic and subatomic scales. Wave–particle duality is a central feature of its account of measurement and behavior.
Photoelectric effect: The emission of electrons from a material when it absorbs light above a threshold frequency. Its energy thresholds supported the idea that light transfers energy in discrete quanta.
Classical wave: A disturbance that propagates through a medium or field and can spread across space. Unlike a classical wave, a quantum object can produce localized, indivisible detection events.
Quantum superposition: A quantum state formed from a linear combination of possible states, with probabilities determined by measurement. A particle can occupy alternatives whose amplitudes produce wave-like interference.
Probability amplitude: A complex-valued quantity whose squared magnitude gives the probability of a measurement outcome. Amplitudes combine to create interference before detection probabilities are calculated.
Planck’s law: A formula describing the spectrum of thermal radiation, derived using quantized energy exchanges. Its quantization began the chain of ideas that made light’s particle aspect unavoidable.
Classical particle: An idealized localized object with definite position and trajectory at each moment. Quantum interference resists describing particles as always following definite classical paths.
Wave function: A mathematical description of a quantum system that encodes probability amplitudes for measurement outcomes. Its amplitudes explain interference, while measurements yield definite outcomes.
Interference: The reinforcement or cancellation produced when waves or quantum amplitudes combine. Interference is the clearest observable signature of wave-like quantum behavior.

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